Many dolomite units in the early to middle Paleozoic Cordilleran miogeocline of Nevada formed through diagenetic replacement of subtidally deposited CaCO3 by CaMg(CO3)2. Across the miogeocline, the consistent temporal and spatial distribution of limestone on the west and dolomite on the east indicates that there was a paleogeographic control on diagenetic dolomite formation. Dolomitization models involving hypersaline brines are inadequate to account for the origin of regionally extensive replacement-dolomite formations in Nevada that lack associated evaporite-mineral suites.
Several workers have recently proposed that the mixing of marine pore water with meteoric-derived ground water would lead to dolomitization of CaCO3 in the subsurface, without precipitation of evaporites. We propose that subtidally deposited CaCO3 in east-central Nevada became dolomitized in the subsurface as a result of dilution of marine pore water by fresh ground water derived from subaerially exposed tracts in the eastern part of the miogeocline. CaCO3 sedimentary deposits in west-central Nevada were never intruded by fresh water because they were not proximal to areas of freshwater recharge; consequently, these sediments escaped dolomitization. The freshwater-seawater-mixing model is compatible with the regional extent of replacement-dolomite formations in Nevada and resolves the relation between dolomitization and paleogeography.